1. Field of the Invention
This invention relates to a liquid crystal display device, and more particularly, to a liquid crystal display device comprising a dynamic scattering liquid crystal cell formed by a nematic or long-pitched cholesteric crystal oriented in a twisted pattern between a pair of substrates carrying transparent electrodes thereon, and polarizers.
2. Description of the Prior Art
Display devices employing liquid crystals have of late been put to practical use at a fast rate. Above all, reserach and development work is actively under way for matrix liquid crystal display devices because of the diversified possibilities of their application. Portable electronic calculators are beginning to be available on the market.
There are known a number of types of matrix liquid crystal display devices, including TN (twisted nematic), DAP type utilizing variation in the rate of double refraction, DS (dynamic scattering) type utilizing the effect of light scattering, and Ch-N (cholesteric-nematic) phase transition type. TN and DAP type liquid crystal display devices, which utilize the double refraction of a liquid crystal, provide a disadvantageously narrow viewing angle. While the viewing angle is not so strictly restricted in DS or phase transition type devices, they disadvantageously fail to maintain a visible display when viewed along the axis of a light source. Anyway, none of these types is considered to constitute a very satisfactory matrix display device. There has, therefore, been proposed recently a DTN (depolarization in twisted-nematic) cell device comprising a pair of polarizers, a DS cell composed of a nematic or long-pitched cholesteric liquid crystal twisted at an angle of 90.degree., and a diffuser.
A display device employing a DTN cell utilizes the depolarization of light by a DS cell upon application of voltage thereto. It is reported that this device can provide a sharper rise in contrast in the vicinity of a threshold voltage, and a much wider viewing angle than a device employing TN cell, and is suitable as a matrix display device having a large number of scanning electrodes (Tatsuo Uchida, Yutaka Ishii and Masanobu Wada: "Properties of a Display Device Using Depolarization in a Twisted Nematic Liquid-Crystal Layer", Proceedings of the SID, Vol. 21/2, 1980, pages 55 to 61).
If a matrix display device employing a DTN cell is used for displaying an image signal, such as a TV picture, using a driving voltage of which the waveform does not only contain a low frequency, but also a considerably high frequency component, it is necessary to raise the cut-off frequency of the liquid crystal composition to a considerably higher level than the high frequency component in the driving voltage, without impairing the response of the low frequency component to the voltage.
In view of these problems, the inventors of this invention have found it useful to incorporate a specific quantity of dopant into a liquid crystal layer, and to apply a specific agent for molecular orientation to an insulating layer. As a result, they have succeeded in obtaining a liquid crystal display device which constitutes this invention, and is suitable for the display of an image signal.